The classic ontological argument

[The Grand Nudger]

Concepts exist, and can have attributes, but telling me that a thing has x y and z attributes and also exists tells me nothing new about the concept of the thing - which does exist by virtue of it's mere proposition and which is what it is and has those attributes proposed regardless of whether or not it's instantiated in the real world (or is even a logical proposition). Rules that pertain to the truth of judgements do not always pertain to the existence or state of existence of things (and vv). This is a fancy way of saying, among other things, that it's possible for us to have good reasons to believe in things which do not exist, and good reason to disbelieve in things that do. It also means that what we may apprehend as a good reason from one category or modality is a uniquely bad one in another. This is why some people perceive such arguments to be compelling. The list of logical fallacies is an ongoing accounting of human cognitive ticks. The platonists tried to handle this with their ideas about the four or five essential elements, kant called it modality, and in the 40's it became category.